gutter determines the space between columns. If the border setting is used, it must be less than or equal to the gutter.
gutter is an integer value and defaults to 5
Gutter 15px, border 3px, with captions
[featured_image_pro gutter=15 border=3 showcaptions=true margibottom='10px']
You can also go for a tight look by setting both gutter and margin to 0.
Gutter 0px (with Margin 0px)
[featured_image_pro gutter=0 marginbottom=0]